Mrs. Maier lives alone in her small house by the lake. She remains unbothered by anything or anyone until one ordinary Monday morning she discovers a woman's corpse right at the shore by her house. Now, nothing is as it was: a nighttime visitor lurks around her home, leaving eerie messages, the sister of the murdered woman is threatened, and another corpse is not long in coming. As the police stand by doing nothing, Mrs. Maier takes the investigation into her own hands, putting herself in grave danger.